1. Field of the Invention
Aspects of the present invention relate to a lithium ion battery, and more particularly, a lithium ion battery having an ultra slim and large size electrode plate to form an electrode assembly which does not require piling an electrode plate in layers or winding an electrode plate in layers.
2. Description of the Related Art
Recently, as improvements have been made to make electric instruments and portable wireless products, such as a video camera, a cellular phone, a lap-top computer and a PDA, more lightweight and compatible with each other, it has become increasingly important to develop improved batteries that are used as power supplies for these products. As a result, research about developing these improved batteries is increasing.
Specifically, a lithium ion secondary battery can supply an electric charge and discharge and has a high energy density per unit weight as well as quick charging capability compared to a lead storage battery, a nickel-cadmium battery, a nickel-hydrogen battery and a nickel-zinc battery, because of the light characteristics of the lithium atom. Thus, researchers are actively researching and developing improved lithium ion secondary batteries.
The lithium ion secondary battery uses a non-aqueous electrolyte to prevent an adverse reaction from occurring between the lithium and moisture. The electrolyte may be a solid polymer including a lithium salt, or a liquid dissolved lithium salt derived from an organic solvent. The liquid dissolved lithium salt is usually an ethylene carbonate, a propylene carbonate, a carbonate including a different alkyl group or a similar organic compound. The liquid dissolved lithium salt has a boiling temperature over 50 degrees and very low vapor pressure at room temperature. The lithium ion secondary battery can be divided into various types, including a lithium metal battery, a lithium ion battery which uses a liquid electrolyte, and a lithium ion polymer which uses a polymer solid electrolyte. The lithium ion polymer battery can be classified into various types, including a pure solid type lithium ion polymer battery which does not have an organic electrolyte, and a lithium ion polymer battery which uses a gel typed polymer electrolyte including an organic electrolyte produced by a different kind of a polymer solid electrolyte.
A pure solid type lithium ion polymer battery does not leak an organic electrolyte, but the gel typed lithium ion polymer battery having an organic electrolyte leaks an organic electrolyte. However, compared to the lithium ion battery which uses a liquid electrolyte, the leaking problem of the lithium ion polymer battery can be reduced. For example, a lithium ion polymer battery may include a multilayer film pouch made from metal foil and more than one polymer film covering the foil, instead of the conventional metal can used with a lithium ion battery. When a multilayer film pouch is used, the weight of battery is reduced compared to the weight of the battery which uses the metal can. Normally, aluminum is used as the metal from which the foil in the multilayer film pouch is made. The polymer film formed as an inner layer of a pouch film inner layer can prevent contact between the metal foil and an electrolyte and thus prevent a short circuit from occurring among the positive electrode, negative electrode and electrode tabs.
The conventional film pouch is made according to the following method. The first operation is forming an upper part and a lower part of the pouch by folding the middle part of a rectangular pouch film formed integrally with the lithium ion battery based on the length direction of a side. The next operation is forming a groove to receive a battery cell through a pressing part in the lower part of the pouch film. The next operation is winding the multilayer film which is stacked in the order of a positive electrode plate, a separator, and a negative electrode plate, and then forming an electrode assembly out of jelly roll type. A separator is added to an electrode plate that is disposed on the external side of the electrode assembly to prevent the short circuit of a positive electrode and negative electrode when the electrode assembly of jelly roll type is formed. The next operation is disposing the electrode assembly made out of jelly roll type on the groove of the lower part of the pouch film. The final operation is sealing the edges of three sides by adding heat and pressure, without sealing the one side having the pouch film among the four sides of the lower part of the groove.
However, in the case of the lithium ion secondary battery of the multi stacked type or the multi winding type as described above, the size of a battery capable of development and mass production is limited. Thus, in order to use an oversized battery, a battery pack must be used, which involves connecting plural batteries together in series or in parallel. The battery pack has various problems, including the fact that an additional process is needed to construct the battery pack, the unit cost needed to construct an electric circuit and battery pack increases, and the total volume of the battery increases substantially.